The present invention relates to an electron source, in particular for a RHEED measurement system, as well as a RHEED measurement system as such, and a method of radiating an electron beam with an electron source onto a sample.
The diffraction of electrons on a crystal surface is in many aspects similar to the optical diffraction of light or X-rays. The Bragg's diffraction law applies for both and governs the appearance and size of the diffraction diagram. The two essentials parameters of the incident beam are found in Bragg's diffraction law, i.e. its wavelength and the incidence angle. As the wavelength associated to electrons is small and comparable or smaller to the lattice spacing of atoms in crystals, electron diffraction, as X-ray diffraction, is used to analyze the atomic structure of materials. Because electrons are strongly absorbed, they cannot penetrate or be transmitted through the sample. Unlike for X-ray diffraction, the electron beam diffraction is performed by reflecting the electron beam onto the surface. Two major diffraction techniques have been developed, depending on the incidence angle of the electron beam: at large incidence angle, normal incidence for instance, the suitable wavelength is obtained at lower electron energies in the range 10 eV to 300 eV. This technique is called LEED for Low Energy Electron Diffraction. At grazing incidence angle the energy can be much larger, in the range 10,000 eV to 60,000 eV. This technique is called RHEED for Reflection High Energy Electron Diffraction. Most commonly, the incidence angle is in the range 2 to 4 degrees only. A precise and stable adjustment of the incidence angle is mandatory. Re-adjustments of the incidence angle are frequently needed when changing the beam energy or the orientation of the sample.
The RHEED technique has recently grown to be a major investigation tool for monitoring crystal growth in vacuum chambers. It is commonly used to control in-situ during the growth process the quality and thickness of the material deposited under good vacuum conditions with pressure in the range 10−6 to 10−11 Torr.
A conventional set-up for RHEED is given in FIG. 7: a high energy electron emitter 5′ produces a dense, well collimated beam 7′. The beam impinges the surface of a sample 1′ at low incidence angle. The beam 7′ is diffracted according to the crystal structure of the surface and the diffraction diagram is observed on a fluorescent screen 20′ mounted inside the vacuum chamber on the other side of the sample.
The high energy electron emitter 5′ is mounted by means of flanges 3′ and 4′ onto the vacuum chamber 2′. The axis of flange 3′ can have various orientations depending on the chamber design: it usually points towards the centre of sample 1′, but the axis is also often parallel to the surface, as shown in FIG. 7. A deflection stage (electrostatic or magnetic) 6′a, b is used to adjust the beam onto the sample 1′. The deflection should be in two perpendicular directions X and Y. The deflection stage consists of separate units 6′a and 6′ b for X and Y orientation. The deflection stage can be located inside (electrostatic or magnetic) or outside the electron source (magnetic). Optionally, a mechanical device consisting of a vacuum bellow 8′ and adjustment screws 9′ may be added to adjust the orientation and position of the electron source with respect to the sample.
This conventional design, with or without mechanical stage, is the most commonly used RHEED set up. It is however strongly limited for the following new applications in particular in wafer production devices and in high pressure environment:
Wafer production machines for MBE, CVD, etc. have a large vacuum chamber. The distances travelled by the electron beam are in the meter range instead of the decimetre range. The beam position and stability is much more affected by residual magnetic fields, such as the earth magnetic field and by AC magnetic field generated by the equipment around the beam (mostly at the frequency of the main power line) and the stray magnetic fields generated by other components of the system (such as magnetron evaporation sources). The effect of the mere earth magnetic field (about 0.6 gauss) becomes important: a 35 kV electron beam travelling 500 mm inside a vacuum chamber follows a circular path. The beam is deflected from its original axis and its orientation angle is changed. The deflection angle is as large as 2.7 degrees and beam off set distance is 12 mm at the sample position. Similarly, an AC field of 80 mGauss will broaden the beam (defocusing) increasing the beam spot size up to 3 mm.
Many growth equipments work at higher gas pressure in the chamber, and often use reactive and/or toxic substances. The electron source uses a filament heated to temperatures in the range 800° C. to 1800° C. to thermally emit electrons. The vacuum inside the electron source has to be kept as good as possible in order not to damage the filament (by evaporation and burning). The filament will be also damaged from ions produced in the gas inside the source.
The production machines run over months without stopping or venting the chamber. Huge amounts of material is evaporated and deposited. Some of this material reached the electron emitter and is deposited onto parts of the electron source. The described system will also be used in such harsh environments, especially when hazardous elements are involved, to protect the electron optical system. Besides increasing the reliability of RHEED source, it greatly reduces the amount of contamination on the cathode parts needed to be exchange regularly.
Accordingly, the main problems of the conventional systems result from: 1—a large distance between sample 1 and flange 3, 2—a higher pressure into the chamber and 3—the presence of stray magnetic fields created by some devices in the chamber.
In view of the above problems, an improved design has been proposed for operation at higher pressure (see J. H. Guus et al. in “Appl. Phys. Lett.”, vol. 70, 1997, p. 1888-1890). As shown in FIG. 8, a differential pumping capability is added to the electron radiation system. A small aperture 10′ (0.1 mm to 2 mm) is inserted between the emitter 5′ and the vacuum chamber 2′. A pumping port 11′ is added to differentially pump the emitter volume. Further, an additional differential aperture 13′ is added. This aperture is located close to the sample to limit the distance travelled by the electron beam 7′ in poor vacuum. This reduces beam absorption and diffusion from the gas in the vacuum chamber. The space in between the apertures 13′ and 10′ is pumped out through the flange 12′. The electron beam from the emitter 5′ is precisely focussed and aligned using the deflection stage 6′a, b on the small aperture 13′. The beam emerges into the vacuum chamber 2′ on the axis of the system.
This is the major limitation of the design. In order to adjust the incidence angle, the complete set-up, electron source, vacuum pipes and vacuum hoses have to be mechanically moved. This is achieved using mechanical positioning devices 9′a, b allowing the translation in X and Y directions (9′b) and a tilt of the axis (9′a). The amplitude of displacements is limited by the size of the inner tube of flange 3′. Then, the sample has to be moved also in order to optimise the incidence angle. However, many vacuum systems have a mechanically fixed the sample position and the beam alignment is not possible using this design. Further the mechanical adjustment of the incidence angle is not an easy operation. Owing to the weigh of the electron source (10 to 25 kg), an the vacuum pipe connections to the pumps used for differential pumping, a very sturdy vacuum bellow system 8′ must be used to precisely keep the assembly in position.